Field of the Invention
The invention relates to an electrostatic sprayer device for spraying products in powder form and in particular coating products entrained by air, the invention being more particularly directed to an improvement which makes it possible to prevent a counter-electrode (the particular function of which is to capture free ions from the air-powder mixture ejected towards an object to be coated) becoming progressively less efficient as it is gradually covered with the coating product forming an insulative film.
Electrostatic powder coating is routinely used in industry, in particular to coat objects with a paint applied in powder form to be subsequently melted by application of heat. When such objects are entirely coated with a sufficient layer of powder, they are routed to an oven in which the powder coating is converted into a homogeneous and strong paint layer as a result of the powder particles melting together, generally followed by polymerization.
For electrically charging the particles of the air-powder mixture an electrical field may be created between a charging electrode placed in the air-powder mixture and a counter-electrode separated from the charging electrode by a distance of a few centimeters. It is also necessary to create a deposition field between the sprayer device and the object to be coated. This is generally achieved by applying a high voltage to the charging electrode and grounding the object. The counter-electrode may itself be grounded or held at an intermediate voltage to create the required field given the dimensions of the sprayer and in particular the distance between the two electrodes.
From this point of view, one requirement is to develop devices using voltages that are not excessively high. In particular, it is desirable to create a charging field over a distance of 30 to 40 mm with a potential difference between the electrodes in the order of 30 kV.
It has been observed that if free ions travel with the air-powder mixture towards the object to be coated, being entrained by the deposition field, differences in the concentration with which the powder is deposited can arise between upstanding and recessed parts of the object. These differences in powder concentration obviously give rise to differences in the thickness of the layer of paint after entry to the oven. It is therefore desirable for substantially all of the free ions which have not served to charge the powder to be recovered by an ion trap, in this instance the counter-electrode. Various configurations have been proposed for the counter-electrode for optimum "recovery" of free ions. Many of the proposed solutions place the counter-electrode inside the air-powder mixture pipe or in direct communication with the interior of this pipe. These solutions are generally somewhat unsatisfactory because the powder tends sooner or later to cover the counter-electrode producing an insulative coating which prevents it fulfilling its function as an ion trap. It is assumed that one of the phenomena involved is local melting of the powder particles resulting from the energy with which they impact on the counter-electrode. To prevent such collection of powder on the counter-electrode it has been proposed to clean it continuously by circulating clean air or even to provide a porous counter-electrode through which air is passed continuously. A solution of this kind is described in U.S. Pat. No. 4,039,145, for example. It has also been proposed to place the electrode outside the air-powder mixture pipe but as yet it has not been possible to prevent particles of powder entrained by vortices covering the counter-electrode. In this line of thinking, U.S. Pat. No. 4,228,961 proposes a structure with an axial cylindrical counter-electrode cleaned by a jet of air passed continuously through a very small annular gap around the counter-electrode. Continuous cleaning of the counter-electrode by this air jet has not yielded good results and this failure may be attributed to the fact that the air jet may itself favor the creation of vortices in the vicinity of the counter-electrode. Moreover, it seems that the shape of this counter-electrode and the small cross-section of the passage that the free ions have to take to reach the counter-electrode significantly reduce the effectiveness of the latter as an ion trap. Also, a passage with a cross-section this small can become clogged in use.
The invention proposes a new arrangement for an electrostatic sprayer device for spraying products in powder from comprising a counter-electrode external to the air-powder mixture pipe whose shape and dimensions enable it to function efficiently as an ion trap, said counter-electrode being associated with blowing means serving, among other things, to clean it continuously. The invention is particularly directed to an arrangement which makes it possible to prevent vortices in the air-powder mixture in the immediate vicinity of the counter-electrode, notably because of the movement of the air intended to clean the counter-electrode.